1. Field of the Invention
This invention relates to a method and apparatus for solvent regeneration. In one aspect, this invention relates to membranes for solvent regeneration. In one aspect, this invention relates to CO2 stripping from CO2-loaded solvents.
2. Description of Related Art
Numerous chemical processes, energy conversion processes, and energy utilization processes produce fluid streams containing acid gases, such as CO2, H2S, SO2, CS2, HCN, and COS. These fluid streams may be gas streams such as natural gas, refinery gas, synthesis gas, flue gas, or reaction gas formed in the processing of waste materials comprising organic substances. Removal of the acid gases is typically required to meet environmental emissions regulations, to protect downstream catalysts for chemical processing applications, or to meet the requirements of other downstream processes and may be achieved by a variety of conventional technologies and methods.
Current processes for removing these gases include countercurrent absorption with a regenerative solvent in an absorber column in which acid-gas-rich solvent from the absorber bottom is stripped of its acid gas by applying heat through reboiling. In this process, a variety of chemical solvents including primary, secondary and tertiary amines and potassium carbonate as well as physical solvents that have a high affinity for acid gas such as methanol, N-methylpyrrolidone, propylene carbonate, and tributyl phosphate are used.
Carbon capture and storage technologies offer great potential for reducing CO2 emissions and, in turn, mitigating global climate change without adversely influencing energy use or hindering economic growth. CO2 capture and sequestration is increasingly becoming a supporting technology component of clean coal projects, such as coal gasification facilities, to reduce the overall environmental impact of coal utilization.
Membranes suitable for use in CO2 capture applications are known to those skilled in the art. The membrane properties affecting the separation process include, but are not limited to, membrane material, composition, thickness, density, hydrophilicity, hydrophobicity, porosity, pore size, and pressure drop through the membrane.
The membranes may be polymeric-based, inorganic-based, porous, nonporous, supported or unsupported, depending upon the particular application. In the scientific literature, facilitated transport membranes, such as an immobilized liquid membrane, that is, a supported liquid membrane in which a low-vapor pressure liquid is immobilized in a porous substrate, have also been reported. The advantages of facilitated transport membranes over conventional polymeric membranes include higher permeabilities for reacting species like CO2 and the resultant high selectivities over non-reacting species like N2. This is due to the facilitating reaction mechanism in facilitated transport membranes compared to the solution-diffusion mechanism in polymeric membranes. Facilitated transport membranes are particularly attractive at low reacting species concentrations where the driving force is very low, and are especially beneficial for removing low concentrations of CO2. However, commercialization of immobilized liquid membranes has been limited due to the inherent limitation of stability of the liquid membrane caused by the absence of any chemical bonding to support the matrix, evaporation of the solvent liquid into gas phases during operation, and lower breakthrough pressures.
In the United States, about 40% of all CO2 emissions are produced by electric generating power plants. Existing coal-fired power plants account for about 85% of the total CO2 emitted by all power plants. The membrane requirements for CO2 separation from post-combustion flue gas may be different from CO2 removal from a gaseous stream containing one or more acid gases because flue gas is hot, dilute in CO2 content, near atmospheric pressure, high in volume, and often contaminated with other impurities (O2, SOx, NOx, and ash). The conventional gas separation membrane process operates by a solution/diffusion mechanism, and the separation driving force is provided by the partial pressure difference of each component across the membrane. This process requires either flue gas compression, permeate sweep, application of permeate side vacuum, or a combination of these steps to provide the separation driving force required. Elaborate process design and optimization become prerequisite for conventional membrane processes in CO2 capture from flue gases. The main limitation of conventional membrane processes is the process pressure ratio (feed gas pressure/permeate gas pressure) limitation. When the membrane separation process is pressure ratio limited, the product CO2 concentration will be limited even when the membrane selectivity is much larger than the pressure ratio. Thus, multiple membrane stages are required to generate greater than 95% pure CO2 product from flue gases using the conventional membrane process.
The membrane contactor process (also known as hybrid membrane/absorption process) combines advantageous features of both absorption and membrane processes to provide a cost-effective solution for CO2 capture from flue gases. In this process, CO2-containing gas passes through one side of the membrane while a CO2 selective solvent (typically an amine solution) flows on the other side. CO2 permeates through the membrane and is absorbed in the solvent. The CO2 rich solvent is then regenerated and again fed to the membrane absorber.
Conventionally, the solvent regeneration is achieved a desorption tower with columns. The stripper unit usually operates at slightly above atmospheric pressure and high temperature. The specific surface area per volume for conventional columns are usually between 3.0 and 500 m2/m3 (free dispersion columns: ˜3.0-35 m2/m3, packed and tray columns: 30-300 m2/m3, mechanically agitated columns: ˜160-500 m2/m3). In contrast, a membrane module can provide a specific surface area per volume as high as ˜1,000-9,000 m2/m3, which is up to two orders of magnitude greater than conventional contactors. Therefore, another approach for regenerating the CO2 rich solvent is to use a membrane desorption module operated in a reverse mode to membrane absorption.
Compared to the extensive studies on CO2 absorption, there are relatively scarce literature information on the development of CO2 stripping (desorption) despite the fact that the stripping unit is responsible for the main operational cost of the absorption/desorption process. Lack of such studies in the literature is mainly due to poor thermal and chemical stabilities of commercial available membranes or/and their low mass transfer coefficient at solvent regeneration conditions.
Studies related to membrane desorption reported in the scientific literature have used non-porous (dense) polymeric membranes. In this case, the CO2 desorbs from the solvent on one side of the membrane, diffuses through the polymeric layer and then passes through the porous support for recovery on the other side of the membrane. Kumazawa [Chem. Eng. Commun. 182 (2000) 163-179] investigated CO2 desorption from CO2 loaded 2-amino-2-methyl-1-propanol solution using the membrane desorption process. Non-porous Polytetrafluoroethylene (PTFE) hollow fiber membranes were used in the experiments. Khaisri et al. [Journal of Membrane Science 376 (2011) 110-118] conducted membrane contactor based CO2 stripping from CO2 loaded monoethanolamine solution also using non-porous PTFE hollow fiber membranes. Trusov et al. [Journal of Membrane Science 383 (2011) 241-249] used hydrophobic dense glassy polymers with an excess of free volume fraction, such as poly[1-(trimethylsylil)-1-propyne] (PTMSP), poly[1-(trimethylgermil)-1-propyne] (PTMGP) and poly[4-methyl-2-pentyne] (PMP) in the membrane contactor for the regeneration of physical and chemical solvents in acid gas removal processes.
There appears to be no disclosed information of using porous membranes operated in a “wetted” mode (at least a portion of the membrane pores are filled by liquid feed) for solvent regeneration in the scientific literature. According to conventional membrane contractor theory, care must be taken to prevent the solvent from entering the pores of the porous support because a stagnant liquid film in the pores of the membrane effectively blocks the passage of CO2 through the porous support. This can be accomplished by maintaining the pressure on the feed side of the membrane sufficiently low and the pressure on the permeate side of the membrane sufficiently high so as to prevent the incursion of the liquid solvent into the pores of the porous membrane. Additionally, for those instances in which the CO2-rich solvent may be water, incursion of the water into the porous membrane may be prevented by incorporating hydrophobic properties into the membrane design.